To get the N Number for my aircraft, I sent a letter requesting an N number, and an affidavit of ownership. There are not the most intuitive documents to fill out, since the form to get the N number asks for your assigned N number. If you have Office 97 or better, click here to download the letters that I sent out.
Remember you have to use a Public Notary to sign one of them. And you must sign the document in the presence of the notary, you can't sign it and then bring it to the notary.